Collision Textures and 2DRenderTargets in XNA

I started making a Tower Defense Game which I will be sharing soon, including all my trials and tribulations and the theory behind it. Whilst making it I came across an interesting problem, which was how to tell whether or not the user could place the tower where they currently have the cursor. I thought about it for a while and then decided that the best method would be a sort of collision texture whereby you have a texture ‘underneath’ the main texture which outlines the areas where a tower can and cannot be placed (thought this texture is never actually rendered).